(1) Field of the Invention
The present invention is directed to a frequency division multiplex (FDM) guardband communication system for increasing the communication capability of the system and having a negligible effect on the overall system performance. In particular, the frequency division multiplex guardband communication system of the present invention uses the guardbands which lie between the information bands to send additional information. The energy of the new signal added to the individual guardbands is well below the guardband noise level so that interference with the recovery of the main FDM information does not occur. Recovery of the new signal energy can be facilitated through the use of spectrum type coding on the transmit side and matched filtering on the receive side
(2) Description of the Related Art
Frequency division multiplex (FDM) systems for combining voice grade channels for transmission over a common medium employ guardbands between channels to reduce crosstalk and facilitate demultiplexing. These guardbands represent unutilized bandwidths in an FDM system.
In conventional FDM systems, the basic FDM unit is a "group" consisting of twelve voice grade channels. Each channel is assigned a 4 kHz frequency slot and positioned in frequency so that one of its two redundant sidebands is centered in the assigned slot. The channels are then filtered to remove unwanted sidebands and summed to form the basic group. Higher order frequency division multiplexers are used to combine, for example, five groups into a 60 channel "supergroup", five "supergroups" into a 300 channel "mastergroup", etc.
Within the FDM group, each channel consists of an information band and a two region guardband. The first guardband extends from DC to approximately 250 Hz. The second guardband extends from 3400 to approximately 4000 Hz. Voice (or data) occupies the 250 to 3400 Hz band between the two guard regions. After multiplexing, the upper end of the guardband for one channel combines with the lower end of the guardband for the next higher channel in the multiplex output to form a combined guardband region of 850 Hz between channels. The total unutilized bandwidth therefore, for a 12 channel group, is approximately 9400 Hz (11 bands.times.850 Hz). For a supergroup, the amount of an unutilized bandwidth is approximately 47 kHz, and for a mastergroup is approximately 235 kHz.